Becoming 'energy independant' is folly for the rich.

Solar pannels with acces to "the grid" are silly. There is roughly 1.6kWh available in full sun in any given M^2. Best case current panels achieve 20% eff, meaning that BEST CASE one can expect 1.6 * .2 or .8 kWh per M^2 * full sun hours. Life af a solar panel is ~ 20 years. Assuming that there is a grid connection and that all unused kWh can be sold back to the grid at the same rate charged (rarely true) to cover the uasae of a 100kWh/day home one would need to have 125 M^2 of solar panels @ a cost of ~ $100/M^2. The up-front investment would then be $125k for the pannels alone. Actual cost would be ~ $200+k The current cost of energy is about $0.10/kWh which means that current fixed cost ot 100kWh/day energy is $300/month and the "up-Front cost of "going green" is ~$1000/ month for a 30 year rate of 5%. With an expected life of 20 years the expected "pay off" is closer to 50 years, hence the only way to "win" is tax credits... or, "folly for the rich". If you cannot afford a $135k upfront charge, then you cannot afford to use "free" energy. to become "independant of the grid".

a simple way to encourage him to proceed with plans to 'upgrade' his house would be very welcome.
Or is he correct and we should just forget trying to do it?

Woooow someone doesn't like solar.
My 2story house (with many computers and misc. electronic equipment) uses 7kWh-13kWh/day.
Obviously if you do the maths of using solar to power a house using 10x more electricity than the average user, you are going to get a rather unfavourable economic result for solar.

It's obviously this person has no idea whatsoever on this subject, as pretty much every figure he has stated is incorrect. (e.g He has no idea what the correct irradiance is on the Earth's surface)

I'd sure like to tell him where he can shove his house and his 100kWh/day power uasae.

P.S. Tell him that $100/m^2 * 125m^2 = $12.5k and not $125k. And after you have proved it to him using a calculator, slap him for me:)

Staff: Mentor

The thing is, the amount of power your house uses doesn't change the economics at all - if you double the usage and double the cost of the array, you also double the savings by saving twice as much electricity. Thus there is no change in payback.

As for the analysis itself, he's high on the solar flux (it's 1366, not 1600 w/m^2), low on the energy cost (it varies, but it is usually more like $.13, not $.10/ kWh). He screws up his conversion from kW peak to kWh per day, but his calculation would yield about 2 hours a day, average usage. Reality is probably more like 4. Overall, though, the up and down errors mostly cancel and his conclusion comes in in the ballpark of where it should be.

The gist of it is that I figure about a 40 year payback for the full price of the setup. Where people tend to go wrong is in calculating how many hours of use they get out of the array. Most of the time, they are way high in the estimate.

The thing is, the amount of power your house uses doesn't change the economics at all - if you double the usage and double the cost of the array, you also double the savings by saving twice as much electricity. Thus there is no change in payback.

Surely if you go and apply interest on 200k over a 20 year, 30, or 50 year (whatever he is trying to use) term, the amount of power your house uses (and thus the size of the system, thus the total up front cost) matters.. alot. Especially when a factor of 10 is involved.

Also some more points:

He has conviniently assumed that the cost of electricty will stay at $0.10/kWh for the next 20-50 years. Whether this will be true or not I'm not sure, but I would put my money on it increasing over time rather than staying constant

Some people get solar for the good feeling they get when they know they are running their tv's and other appliances partly or wholly on solar power and not from coal/steam. For these people they can justify their purchase without economic reasons

Also, your panels will not instantly become worthless after you purchase them. After 10 years (or whatever) they still have a monetary value, and you can sell them on to regain some of your upfront cost of the panels.
The only reason for this '20 year lifespan thing' is because manufacturers are giving out warranties / guaranteed 80% minimum outputs, etc. for 20 years. This is because the oldest modules at the time when solar started being a more 'for public sales' thing were 20 or so years old. I don't like putting a number to exactly how long a solar panel will 'last', as there are so many factors involved, but I'll just say that a decent panel should outlive the roof you put it on

Staff: Mentor

Surely if you go and apply interest on 200k over a 20 year, 30, or 50 year (whatever he is trying to use) term, the amount of power your house uses (and thus the size of the system, thus the total up front cost) matters.. alot. Especially when a factor of 10 is involved.

No, it really doesn't.

If your system costs $20,000 and you save $1,000 a year, your payback is 20 years.
If your system costs $40,000 and you save $2,000 a year, your payback is still 20 years.

[*]He has conviniently assumed that the cost of electricty will stay at $0.10/kWh for the next 20-50 years. Whether this will be true or not I'm not sure, but I would put my money on it increasing over time rather than staying constant

Yes, his value is too low and yes, it will increase, but that is generally cancelled out by the interest you have to pay when you buy a solar system as part of your house/mortgage.

[*]Some people get solar for the good feeling they get when they know they are running their tv's and other appliances partly or wholly on solar power and not from coal/steam. For these people they can justify their purchase without economic reasons

True. And fortunately for them, they don't ever think about whether they are actually making a difference. One of the reasons solar panels are so expensive is they require a lot of energy to manufacture. How much, I'm not sure, but I'd be willing to bet it is a significant fraction of the cost of the systems.

[*]Also, your panels will not instantly become worthless after you purchase them. After 10 years (or whatever) they still have a monetary value, and you can sell them on to regain some of your upfront cost of the panels.

True, but why would you do that? After 10 years, they haven't paid back yet and you're still paying for them. That's like selling a car while you're still making the payments on it. The question really is: after their 20 or 40 year payback, do they still have any value left?

The only reason for this '20 year lifespan thing' is because manufacturers are giving out warranties / guaranteed 80% minimum outputs, etc. for 20 years. This is because the oldest modules at the time when solar started being a more 'for public sales' thing were 20 or so years old. I don't like putting a number to exactly how long a solar panel will 'last', as there are so many factors involved, but I'll just say that a decent panel should outlive the roof you put it on

Roofs only last 20 years, so that isn't saying much. You're right, though - long the panels will really last certainly is an open question.

<sigh> In the end if energy independence through solar panels was cost effective compared to grid power then everyone would be doing it. Maybe solar panels will come down in cost or up in efficiency, but I'm not holding my breath since most companies that promise big breakthroughs in 6 months go out of business in 24, especially those in green power.

Russ is absolutely right, energy consumption does not matter, unless you can get a discount on your solar panels in bulk. The sad truth is the only people I see going solar are the ones that have the extra cash to spend to make themselves feel good about themselves or people who don't use AC or dishwashers, or any number of the things that make life easier. It's pretty much the same crowd that drive Priuses, another example of something that will never pay for itself for what you get. I am looking at potential solar power for my home, but thats on a hobbyist level, and NOT photo voltaic based because I can do the math.

P.S. Since the panels will need to be replaced in 20+ years (I say 20+ because you will be drawing grid power more and more over the panel life as the panels degrade) and you will be responsible to replace panels that get damaged by hail, hurricanes, kids lobbing stones (either paying out of pocket or through increased insurance premiums.) the cost needs to get well below the break even point before most people will consider it.

P.P.S. When the cost gets low enough or grid power gets high enough, people will start to change their buying habits, but now is not the time, and pushing the prices up through environmental 'concern' or through inaction is like telling everyone driving a ten year old car who complains about high gas prices that they should be driving a prius. Let them eat cake indeed.

P.P.P.S 20% efficiency is a long way off. Try closer to 5-10% for most comercial moduals. (yes top end comercial can come up to 18, but look at the cost!) Don't believe me? Divide 1kw/m2 by the ouput divided by the area of any panel you chose to look at. Yes there are lots of people trying to get 20%+ cheaply, but we don't have it yet.

...
P.P.P.S 20% efficiency is a long way off. Try closer to 5-10% for most comercial moduals. (yes top end comercial can come up to 18, but look at the cost!) Don't believe me? Divide 1kw/m2 by the ouput divided by the area of any panel you chose to look at. Yes there are lots of people trying to get 20%+ cheaply, but we don't have it yet.

This is not about solar for homes, but there is a solar challenge on for the next week. The race goes from Texas to Alberta and the schedule is at:
http://americansolarchallenge.org/event/asc2008/schedule.html [Broken]
The NASC2008 Standings, updated daily:http://www.americansolarchallenge.org/
According to the media some teams spend close to a million dollars on these cars

4kw solar panels. 8hour effective sun per day. 32kwh/day or 960kwh/month with no degradation. or about 15.6 cents/kwh @ $150/month financing. Now assuming that you have no equipment failures or damaged cells after ten years sunpower quotes around $200/month. Now I am really unsure about how they get their figures since if this system is going to cost me the consumer $27000 after 'free' government incentives then the payment on a 30 year loan is about $200/month at 8%. Also given that solar panels degrade somewhere around 25% after 20 years then it will take you a VERY long time to ever see money back on your investment assuming nothing breaks.

But on the other hand you could spend $15000 (or more) on either a solar chiller system or a geothermal air conditioning system (or not use AC at all), replace all your light bulbs with compact florescent that contain mercury, not run your oven/dishwasher/washing machine and live off a smaller solar system. But you could also live in a hut, your choice. In any mater my quality of living does not increase with the current prices of solar, it decreases drastically. Thus energy independence is the folly of the rich. If they were poor then they would not worry about where there power comes from but rather how they can afford it.

4kw solar panels. 8hour effective sun per day. 32kwh/day or 960kwh/month with no degradation. or about 15.6 cents/kwh @ $150/month financing. Now assuming that you have no equipment failures or damaged cells after ten years sunpower quotes around $200/month. Now I am really unsure about how they get their figures since if this system is going to cost me the consumer $27000 after 'free' government incentives then the payment on a 30 year loan is about $200/month at 8%. Also given that solar panels degrade somewhere around 25% after 20 years then it will take you a VERY long time to ever see money back on your investment assuming nothing breaks. ...

Run the calculator at the link I posted for more details.http://sunpowercorp.com/For-Homes/How-To-Buy/Solar-Calculator.aspx
Its fairly clear that in some areas solar does indeed pay for itself over time: in the southern parts of the US without much cloud cover, with high electricity costs, and if the municipality provides substantial tax breaks for PV like southern Ca it can pay off.